Long-circulating liposomes (also known as stealth liposomes) are lipid based nanoparticles that have been extensively studied as drug delivery system for the treatment of cancers. Liposomal drugs improve the outcome of treatment mainly via 1) increasing the amount of drugs accumulated at tumor sites, and 2) decreasing the systemic toxicity that is associated with free drugs. In this application, we propose to further improve current liposomal vectors (stealth liposomes) to facilitate the drug release selectively at the tumor site, leading to an improvement in the bioavailability of chemotherapeutic drugs and thus a further enhancement in anti-tumor activity. Design of this novel, target-sensitive nanodelivery system is based on the following discoveries that: 1) ?PA is a serine protease that selectively recognizes and cleaves specific peptide sequences;2) ?PA is overexpressed with remarkable consistency in many types of malignant human tumors including prostate cancer;3) mouse or human group X secreted phospholipase A2 (sPLA2-X) is highly efficient in hydrolyzing phosphatidylcholine-rich vesicles;and 4) sPLA2-X is synthesized as an inactive proenzyme and becomes activated after the pro-peptide is enzymatically cleaved from the N-terminal end. We propose to generate and incorporate into the surface of liposomes a fusion protein in which a pro-peptide is coupled to sPLA2-X at the N-terminus via a ?PA-sensitive linkage. Such modification is expected to result in inactivation of the enzyme. We hypothesize that sPLA2-X pro-enzyme, upon co-delivery to the tumors with drug-encapsulated liposomes, will be activated by ?PA, resulting in degradation of the liposomes, selective release of the encapsulated drugs at the tumor site, and efficient killing of tumor cells. Accordingly, specific aims of this application are: Aim 1: To develop a ?PA-sensitive liposomal doxorubicin (DOX) and characterize its biophysical and pharmacological properties in vitro. Aim 2: To study the distribution and therapeutic effect of ?PA-sensitive liposomal DOX in an animal tumor model. Successful completion of these studies will lead to development of novel liposomal drugs that will advance the treatment of ?PA-positive cancers.